If $wgAllowExternalImages is false, enabling this will turn on an on-wiki external image whitelist at MediaWiki:External image whitelist (a page on the wiki).
Items on this whitelist page are regular expression fragments that are each matched when an external image is being parsed.
Items that match a whitelist entry will be displayed, those that do not match will be linked.

Lines on the whitelist starting with # and blank lines are not used to find a match